Multiphoton ionization photoelectron measurements for H2S were carried out at several laser wavelengths in the 422–475 nm region, in order to obtain a direct evidence for the mechanism of ionic fragmentation which takes place by resonantly enhanced multiphoton ionization (REMPI). An ion current spectrum of multiphoton ioniztion was also measured for H2S in this wavelength region, indicating that the ionization takes place via three‐photon resonant Rydberg states. From photoelectron spectra obtained here, it has been found that the main peaks are attributed to the H2S+ ion in the ground state with v = 0. Other photoelectron bands due to v = 1 have also been observed together with some satellite bands. It should be mentioned that no photoelectron bands above 1.3 eV have been found. These experimental evidences directly support the parent ion fragmentation mechanism that the formation of HS+ and S+ ions mainly results from the ground state H2S+ ion with v = 0 and v = 1, respectively, by additional photon absorption. This conclusion differs from that recently suggested by Carney and Baer from their mass‐resolved REMPI experiments.

Rotation and rotation–vibration Raman spectra of HD compressed by argon were recorded at room temperature (72 to 695 amagat) and 175 K (95 to 364 amagat). At low densities, an important motional narowing in the Q branch conceals the vibrational broadening, while this last mechanism becomes the main contribution to the bandwidth when pressure is sufficiently increased. Discussion of the results in the impact limit is given for the low density range where a linear density dependence of the rotational linewidths is observed. The validity of second order calculations in a semiclassical approach is discussed. The case of the HD–Ar system is compared to that of pure HD for which new data complete previous experimental studies.

Diode laser spectra of most of the Q branches of the ν9 band of ethane from RQ8–PQ15 have been recorded. The Q branches RQ0–RQ4 were deconvoluted to yield an effective resolution of (0.5–1.0)×10−3 cm−1 FWHM. Torsional splittings were observed for most lines. In contrast to predictions based on first order theory, the splittings which range from (2–53)×10−3 cm−1, have a marked J and K dependence. A second order theory of torsion‐vibration‐rotation interaction between ν9 and 3ν4 is developed, which fits the splittings with an rms error of 0.0006 cm−1, using only three adjustable parameters: the barrier to internal rotation in ν9, the energy difference between ν9 and 3ν4, and an effective coupling constant. The barrier to internal rotation in ν9 is found to be 1123±10 cm−1.

The DEPT pulse sequence (π/2)(H,y)−(2J)−1−π(H), (π/2)(C,x)−(2J)−1 −ϑ(H,x)π(C)−(2J)−1−(acquire 13C) is analyzedtheoretically for a variable ϑ pulse for three spin systems: CH, CH2, and CH3. It is shown that the pulse train produces an enhanced distortion‐free 13C signal which has the following characteristics: (a) there is phase coherency within and between the components of the 13C multiplets; (b) the enhancements vary with ϑ as (γH/γC)sin ϑ for CH, (γH/γC)sin 2ϑ for CH2, and (3γH/4γC) (sin ϑ+sin 3ϑ) for CH3. Experimental evidence is provided for these predictions. An important application of the DEPT pulse train is for the generation of both individual proton‐coupled and proton‐decoupled 13C methine (CH), methylene (CH2), and methyl (CH3) subspectra. This can be readily achieved by forming suitable combinations of DEPT spectra determined at ϑ = (π/4), (π/2), and (3π/4). Such spectral editing is less sensitive to variations in J values than the INEPT pulse sequence. Signal enhancement for 195Pt and 29Si NMR signals are also demonstrated using the DEPT sequence. The only disadvantage of this pulse train compared with the INEPT sequence appears to be its greater sensitivity to spin relaxation, a consequence of its time span.

Evidence of the existences of two stable phases of solid O2PF6 with a phase transition at −67 °C was found by a temperature‐dependent Raman study (0–2000 cm−1) of the polycrystalline material. The dynamics of the O+2 cation were examined in terms of the relaxation times associated with the O+2 stretching modes in the phases. The O+2 ion in phase I (T≳−67 °C) appears to enjoy greater rotational freedom than in phase II. No evidence of an anticipated third phase was found down to −138 °C.

The phosphorescence spectra from the three individual triplet spin sublevels of pyrazine‐d4 are obtained. The radiative rate constants are determined for individual vibronic bands of the individual sublevel phosphorescence spectra. Based on the radiative characteristics of the individual vibronic bands, the vibrational analysis is made anew. The assignments thus accomplished substantially differ from previously reported results. Anomalous behavior of the radiative rate constant ratios for the totally symmetric vibrations ν1 and ν9a is interpreted from the standpoint of the displacement of the potential energy surfaces. The nonradiative decay rate constants are found nearly identical to those of pyrazine‐h4 for any sublevels. The absence of the deuterium effect is discussed in relation to the nature of the accepting mode.

In this paper, we calculate the k→k′ scattering rates of excitons by acoustic phonons. We consider two‐phonon processes only, having considered one‐phonon processes in an earlier paper. We treat one‐phonon events and a single two‐phonon event in both pure and impure crystals. We apply these results to the recent experimental work on tetrachlorobenzene and discuss the dependence of the rates on energy mismatch.

Simultaneous Brillouin‐correlation light scattering results obtained under an external applied stress pulse are presented for annealed and unannealed (as grown) crystals of benzil. Critical fluctuations are characterized for the LA a‐axis mode governed by the elastic constant c11 and are shown to contribute to this elastic anomaly in the high temperature phase. Both a central peak intensity anomaly and a critical relaxation time anomaly are characterized for unannealed crystals. These effects disappear in well‐annealed crystals and without the external stress pulse even in as grown crystals. The critical relaxation behavior in benzil is coupled to the external stress compression wave generated by the mechanical refrigerator used to cool the sample. The latter interaction and relaxation anomaly are observed in polarized scattered light with the stress applied along the b (or a) axis as required for a c11 coupling. These observations are qualitively analyzed employing the theory of anelastic solids with higher order fluctuation terms incorporated into the free energy expansion. The relationship of this description to the Halperin–Varma theory of defect induced central peaks is outlined.

High and low frequency Raman scatteringexperiments have been performed in the whole temperature range of plastic neopentane (140–257 K). High frequency spectra are well accounted for by Larsson’s isotropic two‐step stochastic model when the vibrational contribution is not neglected. The discussion shows that the results are the best for the spectral part where the isotropic aspect of the model is less important. The low frequency spectra are explained by a transient dielectricanisotropy of the molecules induced by their deformation as they overcome a high potential barrier during reorientation movements. The analysis of both spectra provide consistent results for the temperature dependence of the fraction of molecules performing large amplitude rotations.

It is shown that, in an optically active medium, if sum or difference frequency generation or four‐wave mixing occurs, the response to left and right circularly polarized light should be different. Similar effects are studied where, in analogy to the Faraday effect, the optical activity is induced by an external static magnetic field.

The integrated band intensities in the infrared spectra of a series of sp2‐hybridized hydrocarbons— ethylene, allene, and benzene— are interpreted in terms of C–H bond polar parameters using a recently proposed parametric model of IR intensities. The respective C–H bond polar parameters have identical signs in the entire series of molecules. Their magnitude varies within acceptable limits with more distinct differences for some parameters. These differences are attributed to the varying electronic structure of the C–H bonds in the three molecules, reflected in the intensities of the corresponding vibrational bands.

A new test for judging the goodness of estimated decay parameters is presented. The test is based on the fact that a convolution is invariant under exponential depression. In the absence of significant error the estimated parameters will then remain constant as the degree of depression is varied over a finite range. In the presence of error, the parameters will vary. Up to now, no test has existed to see if moment index displacement corrects errors to a satisfactory extent in any given analysis. It has always been necessary to have some apriori knowledge of the type of error that limited the analysis. The test presented here removes that requirement. In addition, it is shown that the test performs better than a visual inspection of residual and autocorrelation plots in judging analyses when decays are closely spaced, even in the absence of nonrandom errors. The test is useful in accepting or rejecting analyses, with or without automatic error correction, in helping to discriminate between different models of sample decay, and in tuning pulse fluorometers for optimal performance. The test is, in principle, independent of the method of moments; it may be used with any method which needs only a small amount of computer time, and which is a statistically resistant procedure.

The lambda invariance test is a requirement that estimated decay parameters to be locally flat as a function of the degree of exponential depression. This paper presents a converse theorem: Suppose one calculates decay parameters as if they represented a multiexponential decay, but without any apriori knowledge that the decay is, in fact, multiexponential. Then, if the parameters are locally flat, the decay is, in fact, multiexponential. It is shown that global flatness is not required. Only local flatness is needed.

The far‐IR spectrum of kappa‐casein, a giant molecular diode, has been obtained in the region 50–250 cm−1 and its electronic conduction properties have been determined. The conductivity is found to be four orders of magnitude higher in the F‐helix form than in a globule.

In this paper we explore the effects of diagonal structural disorder on the optical absorption line shapes for two‐particle exciton–vibron bound states and for exciton–vibron continua. ’’Local‐type’’ two‐particle bound states are drastically broadened by disorder scattering, while ’’band‐type’’ two‐particle continua are only slightly affected by disorder scattering. These results are utilized as a diagnostic tool to distinguish between local‐type and band‐type many‐particle excitations, establishing that the two‐particle singlet exciton–vibron excitation in crystalline and in amorphous films of tetracene and pentacene corresponds to a two‐particle continuum.

In this paper we utilize a two‐particle band model to describe cooperative vibrational excitations and vibrational overtones in molecular crystals of diatomic molecules. The cooperative vibrational excitation involves a two‐vibron continuum, while the overtone corresponds to a two‐vibron bound state. The problem of two‐particle vibrational excitations was shown to be equivalent to the perturbation of a band by a local potential, whose strength is determined by the intramolecular anharmonicity. Model calculations were performed for a three‐dimensional, semicircular density of states, resulting in transparent expressions for the energetics and intensities, which were utilized subsequently to provide a self‐consistent analysis of the available experimental results. In all these cases, the local perturbation is sufficiently strong to ensure the appearance of a bound overtone state outside the two‐particle band. Finally, we consider the effects of structural disorder on two‐vibron states, which provide a diagnostic tool for the identification of two‐particle bound states and of two‐particle continua.

The intense methylene scissoring and rocking bands near 1460 and 720 cm−1 in the infrared spectra of the odd n‐alkanes C17–C27 show doubling in phase II, the so‐called hexagonal phase. This doubling, which is shown to be due to interchain vibrational coupling, is highly dependent on temperature and chain length. A possible explanation for this dependency is offered in terms of a combination of static and dynamic factors associated with more or less constrained rotational/twisting motions of the chains about their long axes.

This paper deals with the theory of double‐beam three‐photon absorption, in which the excitation process involves the concerted absorption of three photons from two laser beams which simultaneously irradiate the sample. Equations are derived for the rate of absorption under various polarization conditions, and the dependence on the angle between the two beams is examined in detail. It is shown how a set of experiments at a suitable angle with specified polarizations allows the determination of five molecular parameters which characterize the symmetry of the excited state. It is demonstrated how the results thereby provide for an unambiguous assignment of the excited state to one of six different symmetry classes. It is also shown that the double‐beam experiment allows access to excited states which are forbidden under single‐beam excitation.

A line shape formalism is derived classically which considers molecules undergoing both collisional and intramolecular relaxation. Results for a model system demonstrate a significant effect on the line shapes when chaos sets in and the intramolecular relaxation becomes important. An experiment on polyatomic molecules is proposed and should isolate the two effects.

The vibrational line shapes expected for a case of ’’excitation mixed valency’’ (defined, in analogy, to ordinary electronic mixed valency, as one exciton moving between two equivalent electronic sites) is studied using a semigroup formalism to include relaxation effects. We include the quadratic electron‐vibration coupling, corresponding to frequency changes. The results demonstrate the line narrowing expected in the rapid exchange limit, but also show that similar behavior may be caused by rapid electronic excitation. The simple model is readily extended to include transfer rates and ordinary (electronic) mixed valency. The relaxation process introduced a new time scale into the problem; when this becomes of the order of other relevant time scales (transfer, vibration, barrier residence) interesting line shape phenomena may be observed.